Discrepancy in text object sizes?
The reason for the discrepancy is the inter-character spacing (obj.data.space_character, where obj is the long text object). There are 5 additional inter-character spaces in the long text object, and if I add them to the short string length x 6 then the lengths almost match. There is still a small discrepancy (of 0.06 mm/batch of 5 Is) which I can't explain, but it's so small that it can be safely ignored for the purpose of this exercise.